How To Fill The Offset With Material?
I like to use array for such surfaces

Then I will apply Arrays and close boundary using Bridge Edge loops

Next I need a subdivided plane, and I'll add cloth simulation on it. I need to add two vertexes at corners in a new vertex group to use as a pin group, so that the surface will appear like it held by corners.

Finally, I will add a surface deform on the lattice, so that it will follow plane deformations.